Transmission_I_06_200909 Work Principle of DWDM 62P

ZTE UNIVERSITY

English Training Manual

(V0610)

Transmission_I_06_200909 Work Principle of DWDM

Course Objectives:

To master the background of DWDM

To master the orientation of DWDM in the transmission network

To master the basic structure of DWDM system

To master the key technologies of DWDM system

To master the differences between the frequencies and

wavelengths in DWDM 40/80/160-channel system

To master the optical specifications, such as power, loss, gain and

SNR

To master the difference between CWDM technology and

DWDM technology, such as wavelength range, channel space,

rate, wavelength quantity, optical amplification, transmission

distance, cost and application.

Reference:

·Optical Wavelength Division Multiplexing System

·Modern Telecommunication Base and Technology

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Contents

1 Introduction to DWDM ............................................................................................................................. 1

1.1 Emergence and Background of DWDM ........................................................................................... 1

1.1.1 Evolution of Multiplexing Technology in Optical Network .................................................. 1

1.1.2 Evolution of Transmission Technology in Optical Network (PDH, SDH and WDM) ........... 2

1.1.3 Orientation of DWDM in Transmission Network .................................................................. 5

1.2 Overview of DWDM Technology ..................................................................................................... 6

1.2.1 Relative Definitions of WDM ................................................................................................ 6

1.2.2 Basic Concepts of DWDM ..................................................................................................... 7

1.2.3 Relationship between DWDM and SDH ............................................................................... 9

1.2.4 Relationship between DWDM and CWDM ......................................................................... 11

1.3 Features and Advantages ................................................................................................................. 12

1.4 Future Trends of DWDM Technology ............................................................................................ 14

2 Key Technologies of DWDM System ...................................................................................................... 17

2.1 Basic Structure of DWDM System ................................................................................................. 17

2.2 Optical Transponder Technology .................................................................................................... 18

2.3 Wavelength Division Multiplexing/Demultiplexing Technology ................................................... 21

2.3.1 Overview .............................................................................................................................. 21

2.3.2 Introduction to Optical Multiplexer ..................................................................................... 21

2.3.3 Main Performance Indices ................................................................................................... 24

2.4 Optical Amplifying Technology ...................................................................................................... 26

2.4.1 EDFA Technology ................................................................................................................ 27

2.4.2 Raman Amplification Technology ........................................................................................ 32

2.5 Supervision Technology .................................................................................................................. 35

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2.5.1 Functions of Optical Supervisory Channel (OSC) ................................................................ 35

2.5.2 Requirements for OSC .......................................................................................................... 36

2.5.3 Implementation of OSC ........................................................................................................ 36

3 Relative Technical Standards ................................................................................................................... 39

3.1 Open DWDM System and Integrated DWDM System ................................................................... 39

3.2 Operating Wavelength Range .......................................................................................................... 40

3.3 Operating Wavelength of DWDM System ...................................................................................... 42

3.3.1 Operating Wavelength Area of DWDM Systems ................................................................. 42

3.3.2 Operating Wavelength Allocation in DWDM Systems......................................................... 43

3.4 Main Performance Indices ............................................................................................................... 49

Appendix A Abbreviations .......................................................................................................................... 53

Appendix B New Optical Fiber Types ........................................................................................................ 57

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1 Introduction to DWDM

Key points

Basic concepts and background of the DWDM technology

Overview of DWDM technology

Features and advantages of DWDM technology

Future trends of the DWDM technology

1.1 Emergence and Background of DWDM

The basic concepts of optical network are introduced before we start the study of the

Dense Wavelength Division Multiplexing (DWDM) technology. This section describes

the birth and background of DWDM based on two technologies: the multiplexing

technology and transmission technology.

1.1.1 Evolution of Multiplexing Technology in Optical Network

Various transmission mediums are applied in telecommunication networks, such as

twisted pair cables, coaxial cables, optical fiber and electromagnetic waves. The optical

fiber has the characteristics of great transmission capacity, high transmission quality,

low loss, confidentiality and long regeneration distance, etc.

With the constant development of wide-band and high-rate services in the information

era, not only larger capacity and longer distance, but also convenient and rapid

exchanges are needed for optical transmission systems. Then the multiplexing

technology is introduced into optical transmission systems. This technology enables the

transmission of multiple-channel signals through a single fiber or fiber cable with the

broad frequency band and large-capacity features of fibers. In transmission systems for

multiple signals, the multiplexing mode affects the performance and cost of the system

greatly.

The multiplexing technology of fiber optic transmission network has gone through

three development stages: Space Division Multiplexing (SDM), Time Division

Multiplexing (TDM), and Wavelength Division Multiplexing (WDM).


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The SDM technology features simple design and practicability. But it requires that the

quantity of fiber transmission cores must be configured in accordance with the quantity

of multiplexing channels, which results in poor investment profit. The TDM

technology is widely applied, which is the basis of Plesiochronous Digital Hierarchy

(PDH), Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM),

and IP technologies. Its disadvantage is the low utilization ratio of lines. The WDM

technology supports multiple wavelengths (channels) to be borne on one fiber. So, it is

the major means to expand the current fiber communication network capacity.

1.1.2 Evolution of Transmission Technology in Optical Network (PDH, SDH and WDM)

The traditional fiber optic transmission technologies go through three phrases, such as

PDH, SDH, and WDM, as shown in Figure 1.1-1.

Figure 1.1-1 Three Phrases of Optical Communication Development

The traditional fiber optic transmission technologies, such as PDH and SDH, employ

the one-wavelength-in-one-fiber transmission mode. Due to the restriction caused by

the characteristics of their own components, neither the transmission capacity nor the

capacity expansion mode can meet the requirements of the rapid development of

communication networks, leaving the massive bandwidth resources of fibers far from

being fully exploited.

The DWDM technology allows the transmission of multiple wavelengths over a single

fiber, which has become the most economical and practical means for the fiber capacity

expansion. With its unique technical advantages, the DWDM technology becomes a

simple and economical means to expand the fiber transmission capacity in a rapid and

effective manner. It can fully meet the current needs of the network broadband service

development and lays a solid foundation for the development of the future fully-optical

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transmission network.

The development process of PDH, SDH and DWDM, and the interface specifications

of each technology are briefly described as follows.

1. PDH

The early optical transmission system uses PDH, which introduced Pulse Code

Modulation (PCM) digital transmission technology based on the former analog

telephone network. It multiplexes signals at low rate level into high-speed

signals by means of bit stuffing and byte interleaving.

The signals of the primary group of the PDH system adopts the synchronous

TDM mode, and the multiplexing of other high-order groups adopts

plesiochronous (or called asynchronous) TDM mode.

The PDH system includes three kinds of regional rate level standards

respectively for Europe, North America, and Japan, as listed in Table 1.1-1.

Table 1.1-1 PDH Bit Rate

Country/

Region Primary Group Secondary Group Tertiary Group Quartus Group

Europe and

China

2.048 Mbit/s

30 channels

8.448 Mbit/s

120 channels

(30×4)

34.368 Mbit/s

480 channels

(120×4)

139.264 Mbit/s

1920 channels

(480×4)

North

America

1.544 Mbit/s

24 channels

6.312 Mbit/s

96 channels

(24×4)

44.736 Mbit/s

672 channels

(96×7)

274.176 Mbit/s

4032 channels

(672×6)

Japan 1.544 Mbit/s

24 channels

6.312 Mbit/s

96 channels

(24×4)

32.064 Mbit/s

480 channels

(96×5)

97.728 Mbit/s

1440 channels

(480×3)

From early 1970's to 1980's, the PDH system and devices were popularly used

in the digital network. However, along with the developing fiber communication

technology and user's increasing demands for communication services, the

disadvantages of PDH can not be ignored any longer.

1) The compatibility between the three rate standards is not available, which

obstructs the development of international interconnection.

2) There is no worldwide standard optical interface specification. Private optical

interfaces developed by different manufacturers are not compatible with each


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other, which limits the networking flexibility and increases the network

complexity and operation costs.

3) PDH is a multiplexing structure based on the point-to-point transmission. It only

supports point-to-point transmission, but cannot accommodate complicated

networking.

4) The operation, management, and maintenance must depend upon manual digital

signal cross-connection and service-suspension test, which cannot meet the

monitoring and network management requirements of the modern

communication network.

5) Along with the rate increase, it is more and more difficult to implement

multiplexing of high-order groups through the PDH technology, and

requirements of fiber digital communication for large-capacity and super-high

speed transmission cannot be satisfied.

2. SDH

In mid-1980's, the Bell Communication Research Institute in USA put forward

the concept of Synchronous Optical Network (SONET). In 1988, the CCITT

(former ITU-T) accepted the SONET concept, and formed the worldwide

unified technology standard for transmission network, and renamed it as SDH.

The SDH signals use the synchronous multiplexing mode and a flexible

multiplexing and mapping structure. Code streams at different levels are

arranged regularly in the payload of the frame structure. The payload is

synchronous with the network, so the corresponding software can be used to

directly demultiplex a high-speed signal into the low-speed tributary signal at a

time, called “one-step” demultiplexing”.

The rate specifications of the SDH system are shown in Table 1.1-2.

Table 1.1-2 SDH Signal Levels

SDH Level (ITU-T) OC Level (SONET) Line Rate (Mbit/s)

STM-1 OC-3 155.520

STM-4 OC-12 622.080

STM-16 OC-48 2488.320

STM-64 OC-192 9953.280

SDH standardizes the features of the digital signals, such as frame structure,



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multiplexing mode, transmission rate level, and interface code pattern. It

provides a frame that is supported globally, on which a world-class telecom

transmission network has been developed, featuring flexibility, reliability and

easy management. This kind of transmission network is easy to expand and

applicable to the development of the new telecom services. In addition, it makes

possible the interworking between the devices of different manufacturers.

When the transmission rate exceeds 10 Gbit/s, however, the system dispersion

and other negative influences increase difficulty of long-distance transmission.

Furthermore, the SDH system is the TDM system based on single wavelength.

The single-wavelength transmission cannot fully utilize the huge bandwidth of

fibers. Therefore, the WDM (Wavelength Division Multiplexing) technology is

introduced in the backbone network, to greatly enlarge the transmission capacity

of fibers.

3. WDM

WDM (Wavelength Division Multiplexing) technology is a new generation

optical cable technology with very high speed. The WDM technology enables a

single fiber to carry multiple wavelength (channel) systems, converting one fiber

into multiple “virtual” fibers, each of which works on different wavelengths

independently. Due to its economical efficiency and practicability, the WDM

becomes the major wavelength multiplexing technology widely used in current

fiber communication networks.

The Wavelength Division Multiplexing (WDM) technology is adopted in

OADM equipment. This technology is divided into two categories: Dense

Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division

Multiplexing (CWDM).

1.1.3 Orientation of DWDM in Transmission Network

DWDM is a kind of optical fiber communication technology which can transfer the

information containing multiple digital signals over the same fiber. DWDM expands

system capacity by increasing wavelengths in the optical fiber. At the transmitting end,

it combines (multiplexes) optical signals of different wavelengths before transmitting

them; at the receiving end, it separates (demultiplexes) the combined optical signals in

the optical fiber and then sends them to different communication terminals. Briefly

speaking, the DWDM is used to provide multiple virtual optical channels over the


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same physical optical fiber so that it greatly saves optical fiber resources. The position

of DWDM system in transmission network is shown in Figure 1.1-2.

IP

ATM

SDH

SDH ATM Ethernet Others

DWDM

Fiber physical layer

Open optical interface

Figure 1.1-2 Position of DWDM Technology in Transmission Network

1.2 Overview of DWDM Technology

With the DWDM technology, multiple optical carriers with information (analog or

digital) can be transmitted on one fiber, and the capacity of transmission system can be

expanded easily by increasing wavelengths (channels). It combines (multiplexes)

optical signals with different wavelengths for transmission. At the receiving end, it

separates (de-multiplexes) the combined optical signals and then sends them to

corresponding communication terminals respectively. In other words, the DWDM

technology provides multiple virtual fiber channels on one physical fiber.

1.2.1 Relative Definitions of WDM

The WDM technology enables a single fiber to carry multiple wavelength (channel)

systems, converting one fiber into multiple “virtual” fibers, each of which works on

different wavelengths independently. Due to its economical efficiency and

practicability, the WDM becomes the major wavelength multiplexing technology

widely used in current fiber communication networks.

The WDM is divided into three multiplexing modes: 1310 nm/1550 nm wavelength

multiplexing, Coarse Wavelength Division Multiplexing (CWDM) and DWDM.

1) 1310 nm/1550 nm wavelength multiplexing

In early 1970's, this multiplexing technology only used two wavelengths: one in

1310 nm window and the other in 1550 nm window. It implemented



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single-fiber dual-window transmission through the WDM technology, which

was the initial wavelength division multiplexing case.

2) DWDM

The DWDM technology refers to the WDM technology with small spacing

between adjacent wavelengths, with the operating wavelength in the 1550 nm

window. It can carry 8 - 160 wavelengths on one fiber, and is mainly used in

long-distance transmission systems.

Figure 1.2-1 Schematic Diagram of DWDM System

3) CWDM

The CWDM technology refers to the WDM technology with large spacing

(usually no less than 20 nm) between adjacent wavelengths. Generally, the

wavelength quantity is 4 or 8 (18 at most). The CWDM uses 1200 nm - 1700

nm windows.

The cost of CWDM system is lower than DWDM because it adopts non-cooling

lasers and does not need optical amplifying components. The disadvantages of

CWDM are low capacity and short transmission distance. Therefore, the

CWDM technology is applicable to the communication situations with short

distance, broad bandwidth and dense access points, for example, the network

communication inside a building or between buildings.

1.2.2 Basic Concepts of DWDM

DWDM (Dense Wavelength Division Multiplexing) is a kind of optical fiber

communication technology which can transfer the information containing multiple


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digital signals over the same fiber, as shown in Figure 1.2-2. DWDM expands system

capacity by increasing wavelengths in the optical fiber. Due to the limitation of EDFA

used in the WDM, DWDM equipment mainly works in the 1550 nm window at

present.

Figure 1.2-2 Wavelength Spacing of DWDM system

Generally, DWDM equipment is comprised of the following five components:

1. Optical transmitter end

TX1…TXn, the optical transmitters of all the multiplexing channels,

respectively transmit the optical signals (λ1, λ2 …λn, with the corresponding

frequencies as f1, f2…fn) with different nominal wavelengths. Each optical

channel carrys different service signals, such as standard SDH signal, ATM

signal and Ethernet signal. The optical multiplexer (OM) combines these

signals into one beam of optical wave, which will be output by the OBA to the

fiber for transmission.

2. Optical receiver end

After the optical wave in the line fiber being amplified through the OPA, it is

de-multiplexed by the optical de-multiplexer (OD) and then the signals of

different wavelengths are respectively input to the corresponding multiplexing

channel optical receivers, RX1…RXn.



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3. Optical amplifier end

It is located in the middle of the optical transmission section, on which the

optical signals are amplified by optical amplifiers.

4. Optical supervisory channel

In the DWDM system, an independent wavelength (1510 nm) is used as the

optical supervisory channel for transmitting optical supervision signals. The

optical supervision signals carry NE management and monitoring information

of the DWDM system, so as to manage the DWDM system effectively with the

network management system.

5. Network management system

The DWDM NMS is capable of managing optical amplifying units (such as

OBA, OLA and OPA), wavelength division multiplexers, OTUs and the

performance of supervisory channel on one platform. In addition, it can manage

the equipment in terms of performance, fault, configuration and security. The

information of the NMS is carried by the supervision signals in the optical

supervisory channel.

1.2.3 Relationship between DWDM and SDH

1. Relationship between DWDM and SDH on the transmission layer of optical

networks

Both the DWDM system and the SDH system belong to the transport network

layer. They are the transmission means established on the fiber transport

medium. Their relationship of them in the transport network is shown in Figure

1.2-3.


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Circuit Layer (such as ATM and

IP)

SDH Channel Layer

DWDM Optical Channel

LayerOADM

OTM

ADM

DXC

DWDM system

SDH system

Figure 1.2-3 Relationship between DWDM and SDH in Transport Network

The SDH system implements multiplexing, cross-connection and networking on

the electrical channel layer. The WDM system implements multiplexing,

cross-connection and networking on the optical domain.

2. Multiplexing modes of DWDM and SDH for carrier signals

The SDH is a kind of TDM system based on single wavelength (one fiber

transmitting one wavelength channel). When the transmission rate exceeds 10

Gbit/s, the system dispersion and other negative influences will make the

long-distance transmission more difficult.

The DWDM technology simultaneously transmits multiple optical carrier

signals of different wavelengths in the same fiber, fully utilizing the bandwidth

resources of the fiber and increasing system transmission capacity.

3. Capability of DWDM to transmit signals of different types at the same time

The wavelengths used in the DWDM system are mutually separated and

unrelated with the formats of service signals. Therefore, each wavelength can

carry the optical signal with totally different features from the other one. In this

way, the DWDM can implement the hybrid transmission of various signals.

4. Optical interface standards of DWDM and SDH signals

The optical interfaces of SDH devices should accord with the ITU-T G.957 and

ITU-T G.691 recommendation, which does not specify the central operating



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wavelength.

The optical interfaces in DWDM systems must accord with the ITU-T G.692

recommendation, which specifies the reference frequency, channel spacing,

nominal central frequency (central wavelength), central frequency offset and

other parameters of each optical channel. Therefore, the DWDM system can be

either an open system or an integrated one.

5. Integrated application of DWDM and SDH

The transmission capacity of fiber networks can be effectively improved through

the integrated application of DWDM and SDH.

1.2.4 Relationship between DWDM and CWDM

1． Wavelength range of DWDM system and CWDM system

The operating wavelength range of CWDM system depends on the application

fiber. The wavelength range of common fiber (G.652 A&B) is 1470nm-1610nm.

Water peak fiber only transmits the wavelength of 8-channel + 1310 window.

The wavelength range of no water peak fiber (G.652 C&D) is 1270 nm-1610 nm.

18 wavelengths occupy the low insertion loss window of the whole no water

peak fiber.

1550nm window is adopted in DWDM system, and the wavelength range is

1460nm-1625nm.

2． Channel spacing of CWDM system and DWDM system

Channel spacing of CWDM system: 20nm

Channel spacing of DWDM system: 0.4-2nm

3． Single-channel rate of CWDM system and DWDM system

Maximum single-channel rate of CWDM system: 2.5G

Maximum single-channel rate of DWDM system: 40G

4． Wavelength number of CWDM system and DWDM system

CWDM system provides 8-16 wavelengths with bandwidth of each wavelength

at 2.5G. In that case, CWDM equipment can save large numbers of fibers,

especially for the convergence services in the ring network.


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According to different operating wavelength range, DWDM system provides

8-160 wavelengths.

5． Optical amplifier of CWDM system and DWDM system

CWDM system does not adopt EDFA.

DWDM system adopts EDFA, Raman amplifier and pump amplifier.

6． Transmission distance of CWDM system and DWDM system

Since CWDM system doest not adopt EDFA, transmission distance becomes an

important technical specification. The power loss of CWDM system should be

no more than 30dB. In that case, the typical transmission distance of CWDM

system is only 40-80km.

DWDM system implements ultra long non-electrical regeneration transmission

with the range from several kilometers to more than 2000 km, via OTUs with

different types, EDFA, FEC technology, AFEC technology, RZ technology,

EOA and DRA. If several multiplexing sections are cascaded, the transmission

distance can be extended up to 20000km.

7． Cost of CWDM system and DWDM system

Optic modulating of CWDM adopts the uncooled laser with the wavelength

error tolerance of ±2-3nm, while optic modulating of DWDM adopts cooling

laser. CWDM system inherits the feature of smooth upgrade from DWDM

system. The application of CWDM system greatly deduces the cost of the

investment in the early phrase.

8． Application of CWDM technology and DWDM technology

The CWDM technology is applicable to short-distance networks with various

service interfaces and high bandwidth, such as convergence layer, access layer,

private network, and core layer.

DWDM technology is applicable to MAN core layer, trunk core layer.

1.3 Features and Advantages

1. Fully utilizing fiber bandwidth resources and featuring high transmission

capacity



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The wavelengths in the DWDM system are separated to each other, and thus are

capable of transmitting different services transparently, such as SDH, GbE and

ATM signals, to implement the hybrid transmission of multiple kinds of signals.

The DWDM system multiplexes multiple single-channel wavelengths for

transmission in one fiber, greatly saving fiber resource and reducing line

construction cost.

Figure 1.3-1 Huge DWDM Transmission Capacity

2. Super-long transmission distance

Through EDFA and other super-long distance transmission technologies, signals

of multiple channels in the DWDM system can be amplified at the same time to

support the long-distance transmission.

Figure 1.3-2 Ultra-long Transmission in DWDM System

3. Smooth upgrading and expansion


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Since the DWDM system transmits the data in each wavelength channel

transparently and does no process the channel data, the capacity of the system

can be expanded conveniently and practically only by adding more multiplexing

wavelength channels.

1.4 Future Trends of DWDM Technology

1. Higher channel rate

The channel rate of the DWDM system has developed to 10 Gbit/s from 2.5

Gbit/s, and the system at 40 Gbit/s rate is in the phrase of commercial usage and

the technology becomes more and more mature.

2. More wavelengths to be multiplexed

The DWDM system at early phase usually adopts 8/16/32 wavelengths with

channel spacing 100 GHz and the operating wavelength is in C band. Along with

the constant development of DWDM technology, the operating wavelength can

cover C and L bands with the spacing 50 GHz.

ZTE's ZXWM M900 DWDM Optical Transmission System can support the

multiplexing of 160 wavelengths at most.

3. Super-long all optical transmission distance

The initial construction cost and operation cost for the network can be reduced

through extending all optical transmission and reducing electrical regeneration

nodes.

Traditional DWDM systems use EDFA to extend the passive regeneration

transmission distance. At present, this distance can be extended from 600 km to

above 2000 km, through distributed Raman amplifier and enhanced Forward

Error Correction (FEC) technology, dispersion management technology, optical

equalization technology and effective modulation formats.

4. Evolving from point-to-point WDM to full optical network

Many early DWDM transmission systems adopt the networking mode of

point-to-point or chain, which is mainly used for toll backbone. These toll

DWDM transmission systems always use the back-to-back DWDM backbone

transmission structure with 3R (Reshaping, Retiming, and Regenerating). In this



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structure, lots of Optical Transponder Units (OTUs) are used, which leads to

high construction and maintenance cost, low network flexibility, too many

Optical/Electrical/Optical (O/E/O) conversions, low circuit assignment speed,

and high system fault ratio. This structure will not be adaptive to future

automatic switching transmission networks.

At present, in metro area network DWDM transmission networks, OADM

equipment transfer optical signals of different wavelength channel to

corresponding terminal. They can implement the adding/dropping and

straight-through of wavelengths carrying services. The OADM is divided into

the fixed wavelength add/drop multiplexer and the 100% dynamical add/drop

multiplexer (Rearrangable OADM, ROADM). The fixed wavelength OADM

can add/drop 20% - 40% of the input wavelengths; while some can add/drop all

wavelengths. The ROADM has two kinds of structure, the broadcast/selection

one and the demultipexing/cross-connect multiplexing one. The crucial parts of

the ROADM include the cross-connect unit, wavelength disabler, tunable filter

and tunable laser etc. The ROADM can add/drop unfixed wavelengths. The

networking of OADM equipment is flexible, which can implement chain, ring,

and cross networking.

The Optical Cross-Connect (OXC) is the route switch of next generation optical

communication. In the full optical network, it provides these functions:

connection function based on wavelengths, wavelengths add/drop function of

optical channels, leading the wavelength channels for the sake of best utilization

of fiber infrastructure, and implementing protection and restoration on

wavelength, wavelength group and fiber levels. The OXC is set at the important

tandem point of the network, converging different wavelengths input from

different directions and then output signals with proper wavelengths. Through

OADM and OXC, we can construct more complicated ring network. In the next

generation IP Over DWDM telecom/network architecture, the OXC is an

important stage in the future development of WDM technology.

5． IP over DWDM technology

With the rapid development of bandwidth in Internet core network, the data

flow can occupy the capacity of the whole single-channel fiber system without

adopting DWDM technology (at present, the maximum transmission rate of

commercial single-channel fiber system is 40Gbit/s). In that case, IP over


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DWDM technology is the one of the most important trends of network

communication technology.

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2 Key Technologies of DWDM System

Key points

Basic structure of DWDM systems

Optical transponder technology

Optical wavelength division multiplexing and de-multiplexing technologies

Optical amplifying technology

Supervision technology

2.1 Basic Structure of DWDM System

The DWDM system multiplexes several or dozens of optical channel signals with

different nominal wavelengths to one fiber for transmission, with each optical channel

carrying one service signal.

The basic structure of a unidirectional DWDM system is shown in Figure 2.1-1.

Receiver/transmitter of optical supervision

channel

n

2

1

3

G.692

...

OTU

OM OBA OLA OPA

Receiver of optical supervision channel

Transmitter of optical supervision channel

OD

n

3

2

1

...

Optical transmitter Optical receiverOptical regenerating

amplifierTX1

TX2

TX3

TXn

RX1

RX2

RX3

RXn

OTU

OTU

OTU

OTU

OTU

OTU

OTU

OTU = Optical Transponder Unit, OM = Optical Multiplexer

OBA = Optical Booster Amplifier, OLA = Optical Line Amplifier

OPA = Optical Pre-Amplifier, OD = Optical Demultiplexer

Figure 2.1-1 Basic Structure of DWDM System


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2.2 Optical Transponder Technology

Before introducing optical transponder technology, we briefly describe the optical

interface standards of transmission system:

G.957 - optical interface between SDH equipment and system

G.691 - optical interface of the system with optical amplifier and the rate of SDH

single-channel at STM-64

G.692 - optical interface of the multi-channel system with optical amplifier

To DWDM equipment, convert the signal format in compliance with ITU-T G.692

before carrying services:

Frequency requirement:

WDM channel frequency defined in ITU-T G.692 is 192.1THz, and the smallest

spacing is 50G/100G.

Dispersion tolerance requirement:

Take 2.5G system as an example. The electric regeneration spacing of WDM system

can be up to 640 Km, while the electric regeneration spacing of SDH system is only

50-60Km. Hence, the dispersion tolerance requirements of WDM system is much

higher than that of SDH system.

1. Type of optical sources

At present, the semi-conductor optical sources widely used are Laser Diode (LD)

and Light Emitting Diode (LED).

LD is coherence light source, with large in-fiber power, narrow spectral line

width and high modulation rate. It is applicable to the long-distance high-speed

system. The LED is non-coherence light source, with small in-fiber power,

broad spectral line width and low modulation rate. It is applicable to

short-distance low-speed system.

The light source of the DWDM system adopts the semi-conductor laser diode.

2. Modulation modes of DWDM system laser

There are two methods of light source intensity modulation: Direct modulation

and indirect modulation (that is, external modulation).

1) Direct modulation

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Direct modulation means controlling the working current of semi-conductor

laser directly with the electrical pulse code stream, and thus making it generate

the optical pulse stream corresponding to the electrical signal pulse. For example,

when the electrical pulse signal is "1", the working current of the laser is larger

than its current threshold; and then it generates an optical pulse. When the

electrical pulse signal is "0", the working current of the laser is smaller than its

current threshold; therefore it does not generate optical pulse, as shown in

Figure 2.2-1.

Figure 2.2-1 Direct Modulation

The direct modulation mode is simple, with low loss and low cost. But, the

super-speed change of working current will result in modulation chirp easily.

And the chirp will limit transmission rate and distance.

The direct modulation mode is often used in the transmission system composed

of G.652 fiber, with transmission distance shorter than 100 km and rate lower

than 2.5 Gbit/s.

2) Indirect modulation (external modulation)

The external modulation mode refers to indirectly control (modulate) the

continuous light generated by the laser, which is in the continuous light emitting

status, and thus obtaining optical pulse stream, as shown in Figure 2.2-2.

Figure 2.2-2 External Modulation

Therefore, in external modulation case, the laser generates stable high-power


20

light, which is modulated in low chirp. And the external modulation can obtain

the maximum dispersion value much greater than that in direct modulation.It is

applicable to the long-distance transmission system at rate over 2.5 Gbit/s.

At present, common external modulators include Electrical Absorption

modulator (EA) and waveguide Mach-Zehnder (M-Z) modulator.

· EA modulator

It uses absorber controlled by electrical pulse signals to absorb or not absorb the

optical wave transmitted by the continuous-wave semi-conductor laser (CW),

and thus control optical pulse stream indirectly with the electrical pulse signal

stream.

The EA light source features small size, high integration, low driving power and

low power consumption. The maximum dispersion can reach 12800 ps/nm.

· Waveguide M-Z modulator

At the input end, the CW is in continuous wave working status. The optical

wave emitted by it is divided into two equal signal channels by the optical

de-multiplexer, which will respectively enter two optical tributaries of the

modulator. Under the control of electrical pulse stream, the modulator performs

phase modulation to the optical signals. At the output end, two optical tributaries

are combined by the optical multiplexer. When the signal phases in two optical

tributaries are reverse to each other, the optical multiplexer has no optical signal

output; when the signal phases in two optical tributaries are the same, the optical

multiplexer has optical signal output. In this way, the optical pulse stream is

controlled by the electrical pulse stream.

The M-Z light source features high modulation rate, large maximum dispersion

value, and large extinction ratio. Its chirp coefficient can be zero in theory.

However, its disadvantage is that polarization maintaining fiber must be used to

connect the laser and the modulator, because modulation status is related to light

polarization status.

3. Features of DWDM system light source

1) Providing standard and stable wavelength

The DWDM system has very strict requirements for the operating wavelength of

each multiplexing channel. Wavelength drift will cause unstable and unreliable



21

operation of the system.

The common wavelength stabilization measures are temperature feedback

control method and wavelength feedback control method.

2) Providing rather large dispersion tolerance

Fiber transmission may be limited by system loss and dispersion. With increased

transmission rate, the dispersion influence is larger. The dispersion limit can be

solved by using optical fiber cables with small dispersion coefficient or

semi-conductor laser with narrow spectral width. For the optical cables have

been laid, minimizing spectral width of light source devices is an effective

measure to solve the dispersion limit problem.

2.3 Wavelength Division Multiplexing/Demultiplexing Technology

2.3.1 Overview

The optical wavelength division multiplexer and de-multiplexer, also called optical

multiplexer and de-multiplexer, is actually a kind of optical filter.

At the transmitting end, the Optical Multiplexer Unit (OMU) combines the optical

signals with nominal wavelength in each multiplexing channel into a beam of optical

wave, and then transmits it into the fiber for transmission, that is, multiplexing optical

wave.

At the receiving end, the Optical De-multiplexer Unit (ODU) divides the optical wave

in the fiber into optical signals with formal nominal wavelength of each multiplexing

channel, and then inputs them into corresponding optical channel receivers, that is,

de-multiplexing optical wave.

Since the performance of OMU and ODU determine the system transmission quality,

the attenuation, offset and channel crosstalk of them must be small.

2.3.2 Introduction to Optical Multiplexer

Four types of common OMs are briefly introduced below, as well as the OM types

often used in the DWDM systems with different wavelength numbers.

1. Brief introduction to common OMs

1) Grating OM


22

The grating type of OM is an angular dispersion type of device.

Since the optical signals with different wavelengths have different refractive

angles on the grating, it divides and combines the optical signals with different

wavelengths. Its working principle is shown in Figure 2.3-1.

1,2,3,...n

n

Figure 2.3-1 Working Principle of Grating OM

It has sound wavelength selection performance, and is capable of narrowing

wavelength spacing to about 0.5 nm. However, the manufacture of grating

should be very precise which make it not suitable for large-batch manufacture.

So it is just often used for research in the laboratory.

2) Dielectric thin film OM

It is composed of Thin Film Filter (TFF).

TFF consists of dozens layers of dielectric films with different materials,

different refractive indexes and different thickness. One layer features high

refractive index and the other layer features low refractive index; therefore TFF

emerges a passband within certain wavelength range while a stopband within

other wavelength ranges. In this way, the desired filtering performance is got.

The working principle is shown in Figure 2.3-2.



23

1,2,3,...n

Figure 2.3-2 Working Principle of Dielectric Thin Film OM

The dielectric thin film OM is a kind of compact passive optical device with

stable structure, featuring flat signal passband, low insertion loss and sound

channel isolation.

3) Array Waveguide OM (AWG)

AWG OM is the flat waveguide device based on optical integration technology.

Its working principle is shown in Figure 2.3-3.

Figure 2.3-3 Working Principle of AWG OM

Due to compact structure and low insertion loss, it is the best scheme for optical

wavelength division multiplexing/de-multiplexing in the optical transport

network.

4) Coupling OM

It is a kind of surface interactive device with two or more fibers close to each

other and properly melted, which is mainly used as OM. The working principle

is shown in Figure 2.3-4.


24

λ1

λ2

λ3

λ4

λ5

λ6

λ7

λ8

λ1，2，3……

Figure 2.3-4 Working Principle of Coupling OM

The coupling OM can only implement the multiplexing function, with low cost

but large insertion loss.

2. Multiplexer/de-multiplexer in DWDM systems

The relationship between systems with different wavelengths and their

corresponding optical wavelength division multiplexers used is shown in Table

2.3-1.

Table 2.3-1 Relationship between DWDM Systems and Corresponding Optical Wavelength Division Multiplexers

OMD &Wavelength

Type

OM OD

Below 32

wavelengths

40

wavelengths

Above 80

wavelengths

Below 32

wavelengths

40

wavelengths

Above 80

wavelengths

Coupling type √ - - - - -

Array waveguide type √ √ √ √ √ √

Dielectric thin film type √ √ - √ √ -

Grating type - - √ - - √

2.3.3 Main Performance Indices

1. Multiplexing channel quantity

It represents the quantity of optical channels to be multiplexed by the optical

wavelength multiplexer. The channel quantity is closely related to the resolution

and isolation of the device.

2. Insertion loss

The insertion loss is the attenuation effect of wavelength division multiplexer

itself on optical signals and it will affect the transmission distance directly.



25

Different types of wavelength division multiplexers have different insertion loss.

The multiplexer with smaller insertion loss is preferable.

3. Isolation

It represents the isolation degree between multiplexing optical channels in the

optical device. The higher the channel isolation is, the better is the frequency

selection performance of the wavelength division multiplexer. Consequently, the

crosstalk suppression ratio becomes higher and the mutual interference between

multiplexing optical channels becomes lower.

It is meaningful only for the wavelength sensitive devices (TFF type and AWG

type devices). It is not meaningless for coupling devices.

4. Reflection coefficient

It is the ratio between the reflection optical power and incidence optical power

at the input end of the wavelength division multiplexer. Smaller coefficient is

preferable.

5. Polarization Dependent Loss (PDL)

It represents the maximum change value of the insertion loss caused by the

change of optical wave polarization status.

Light is the electromagnetic wave with extremely high frequency, therefore,

there is the problem of wave vibration direction (polarization). For the optical

signals input to the wavelength division multiplexer, their polarization statuses

will not be totally consistent. And the same wavelength division multiplexer has

different attenuation effects on the optical waves in different polarization

statuses. Smaller PDL value is preferable.

6. Temperature coefficient

It represents the central working frequency offset of the multiplexing channel

caused by the ambient temperature change. The wavelength division multiplexer

with smaller temperature coefficient is preferable. Smaller coefficient means

more stable central working frequency of the multiplexing channels.

7. Bandwidth

The bandwidth is a parameter of the wavelength sensitive devices (TFF type and

AWG type devices). It is meaningless for coupling multiplexers.


26

The bandwidth is divided into channel bandwidth -0.5 dB and channel

bandwidth -20 dB.

· Channel width -0.5 dB

It refers to the corresponding operating wavelength change when the OD

insertion loss decreases by 0.5 dB.

It describes the bandpass feature of the OD. A sound bandpass feature curve

should be flat and wide. Greater bandpass value is preferable.

· Channel width -20 dB

It refers to the corresponding operating wavelength change when the OD

insertion loss decreases by 20 dB.

It describes the stopband feature of the OD. The stopband feature curve should

be sharp. Smaller bandwidth value is preferable.

2.4 Optical Amplifying Technology

For the long-distance optical transmission, optical power gradually decreases with the

increasing of transmission distance. The output of the light source laser usually is not

more than 3 dBm, otherwise, the laser life cycle may be unqualified. In addition, in

order to ensure correct signal receiving, the receiving power at the receiving end must

always be a certain value, for example, -28 dBm. Therefore, the optical power becomes

the major factor determining the transmission distance.

Optical amplifier is the technology to solve the problem of optical power limit. Without

the O/E/O conversion, it directly amplifies the optical signals. The classification of

optical amplifier is shown in Figure 2.4-1.

Semi-conductor OA {

Resonance type

Travelling wave type

Fiber amplifier {

Lanthanon doped fiber amplifier

Non-linear optical amplifier

{1550 nm fiber amplifier, for example, EDFA

1310 nm fiber amplifier, for example, PDFA

{Raman fiber amplifier

(SRA)

Brillouin fiber amplifier (SBA)

{

Figure 2.4-1 Optical Amplifier Classification



27

In this section, the EDFA and Raman fiber amplifier are introduced.

2.4.1 EDFA Technology

2.4.1.1 Technical Principle of EDFA

1. Amplifying principle

Erbium (Er) is a kind of lanthanon. In the fiber manufacture process, certain

quantity of Er3+

ions is doped to form Erbium Doped Fiber (EDF). The Er3+

ions

in such fiber will absorb photon energy to make it own energy level change,

which is called stimulation. The light source for stimulation is called pump light

source, and the corresponding transmitting stimulation optical wave is called as

pump light.

The working principle is shown in Figure 2.4-2.

980 nmpump light

1550 nmsignal light

1480nm

1550 nmstimulatedemission

N1

N3~0

N2

Figure 2.4-2 Working Principle of EDFA

The Er3+

ion free from stimulation is at the lowest energy level. When the pump

light is shot in, the Er3+

ion absorbs energy from the pump light and transits

itself to the higher energy level. At the higher energy level, the Er3+

ions are in

instable status, therefore they continuously converge to metastable energy level

in non-radiant transition format, and thus implementing population inversion

distribution. When the optical signals with 1550 nm wavelength pass this

segment of EDF, the metastable particles are transited to the ground status in

stimulated emission format, and then photons which are the same as those in the

incoming signal light are generated. In this way, the optical signals are

amplified.

2. Composition


28

The EDFA consists of the EDF, bump light source, coupler and isolator, as

shown in Figure 2.4-3.

...

Pin

Isolator WDM coupler IsolatorErbium

doped fiber...

Pump laser

Pout

1 2 n 1 2 n

Figure 2.4-3 EDFA Composition

The coupler is used to combine the signal light with pump light. The isolator is

used to suppress the light reflection, to ensure stable working of the optical

amplifier. The pump laser generates pump light source.

3. Main performance indices

1) Gain (G)

It is the ratio between output optical signal power and input optical signal power.

Greater gain means more powerful amplifying capability.

2) Noise Figure (NF)

It is the ratio between the Signal-Noise Ratio (SNR) at the EDFA input end and

SNR at output end.

EDFA noise comes from many ways, such as signal shot noise, internal

reflection noise and Amplified Spontaneous Emission (ASE) noise, which is the

major part of EDFA noise.

Note

ASE is the emission noise caused by the EDFA’s own factors, such as the unbalance

between optical transmitting area and absorption area, the different population

inversion degrees (quantity of ions in stable energy level E2 and the quantity of ions in

ground energy level E1 are different), the gain and working status of the EDFA.

Since the EDFA can amplify both optical signal and noise, the parameter NF

appears which is closely related to the ASE noise of the EDFA. It greatly affects

the system performance, especially the OSNR of the whole system. Smaller NF



29

is preferable, for example, below 5.0dB.

3) Bandwidth

The operating wavelength range of the DWDM system covers C band and L

band. The optical amplifier needs to amplify all the multiplexing channel signals

of the system, so its bandwidth should be wide enough.

4) Gain flatness (Gp-p)

It represents the allowed fluctuation of EDFA gain within the specified working

band range. In order to get sound flatness, the aluminum doped technology or

gain flatness filter is usually used in the EDF.

In the DWDM system, smaller EDFA gain flatness is preferable so as to

minimize the difference between output optical power signals of different

multiplexing channels and facilitate optical power estimation,.

5) Total input/output power range

It is the optical power range at input/output end of the EDFA.

In WDM systems, an EDFA is responsible for amplifying all the multiplexing

optical channel signals in the system. Therefore, its input/output optical power

range should be large enough, especially for WDM systems with lots of

multiplexing channels.

On the other hand, to ensure the gain flatness and low noise performance, the

EDFA should work in small signal working range, that is, the input/output power

range of the EDFA cannot be too large. It is more important that the EDFA

output power cannot be too large in order to avoid fiber non-linear effect. For

this purpose, the optical power of a signal channel cannot be too large. The

proper power should be determined according to the signal rate and the type of

transmission fiber.

6) Input/output optical reflectance

It is the ratio between the optical power at the EDFA input/output end and the

reflection optical power. Greater value is preferable.

4. Importance of EDFA for DWDM system

To ensure the transmission quality of DWDM systems, the EDFAs used in the

DWDM system must have sufficient bandwidth, flat gain, low NF and high


30

output power. Proper gain flatness is especially important, which is special

requirement of DWDM system for EDFA.

2.4.1.2 Classification of EDFA

Depending on the location of EDFA in the DWDM system and pump source types, two

EDFA classification modes are introduced below.

1. Classification by location

The EDFA is divided into Optical Booster Amplifier (OBA), Optical Line

Amplifier (OLA) and Optical Pre-Amplifier (OPA).

1) OBA: It is located behind the OTM or the transmitting light source of

regenerator device, being in the front of the regeneration segment. The OBA is

mainly used to boost the transmitting power so as to extend transmission

distance.

2) OLA: It is located in the middle of the regeneration segment, with the EDFA

inserted directly into the fiber transmission link for amplifying signals. Multiple

OLAs can be equipped in the regeneration segment as required.

3) OPA: It is located between the end of the regeneration segment and the optical

receiving device. The OPA is mainly used to pre-amplify small signals going

through line attenuation, and boost the power of optical signals before entering

the receiver so as to meet the sensitivity requirements of the receiver.

The locations of these three kinds of amplifiers in the optical line are shown in

Figure 2.4-4.

OTM OBA OLA OLA OTMOPA

Regeneration segment

Figure 2.4-4 Locations of Amplifiers in Regeneration Segment

2. Classification by pump source

The pump sources often used now cover 980 nm and 1480 nm, for these two

types of pump sources have high pump efficiency.

The 980 nm pump light source has lower NF; while the 1480 nm one has higher

pumping efficiency and therefore a larger output power is obtainable (about 3



31

dB higher than that of the 980 nm pump light source).

In actual applications of line amplifier, most 8-channel WDM systems use the

980 nm pump source, because the WDM system of G.652 fiber mostly features

dispersion limit other than loss limit. If such WDM system uses the 1480 nm

pump source, the system power attenuation will increase and it is unnecessary to

boost EDFA output power.

WDM systems of more than 16 channels use the 1480 nm pump source instead,

because enormous tributaries decrease the available power range and the pump

source with higher power is necessary. A two-level pump can also be used to

improve the NF and increase the output power.

2.4.1.3 Main Problems of EDFA to Be Solved

The EDFA also introduces in some new problems while solving some problems of fiber

transmission system.

1. Non-linear effect

EDFA amplifies the optical power through increasing the optical power shot into

the fiber. However, it does not mean the greater optical power is surely the best.

When the optical power is increased to certain degree, fiber non-linear effect

will occur. Therefore, in the usage of fiber amplifier, it is required to control the

value of the in-fiber optical power in a single channel.

2. Bandwidth

Bandwidth refers to the range of the optical wavelength which can be amplified

flatly. The operating wavelength range of the EDFA in C band is 1530 nm - 1561

nm, and the one in L band is 1565 nm - 1625 nm.

The gain flatness filter is used inside the EDFA, so that the EDFA has almost the

same gain to each multiplexing optical channel signal within corresponding

wavelength range. The gain fluctuation should be limited within the allowed

range, for example, ±1 dB. Therefore, the bandwidth is closely related to the

gain flatness.

3. Optical surge

When the optical line is normal, the erbium ions stimulated by the pump light

are carried off by the signal light, thus implementing the amplification of the


32

signal light. If the input light is interrupted, the metastable erbium ions still

converge continuously, and finally the energy transient occurs, leading to optical

surge.

The solution of optical surge is to implement Automatic Power Reduction (APR)

or Automatic Power ShutDown (APSD) function in the EDFA. In other words,

the EDFA should automatically reduce power or shut down power upon no input

light, and thus suppressing surge.

4. Dispersion

With the extended transmission distance, the total dispersion increases

correspondingly. Therefore, the passive regeneration segment in the WDM

system cannot be prolonged limitlessly. The dispersion compensation measure

can be taken to prolong the passive regeneration distance of the multiplexing

section.

2.4.2 Raman Amplification Technology

2.4.2.1 Working principle

The Raman amplification technology bases on the non-linear effect -- Stimulated

Raman Scattering (SRS), that is, when a light wave strong enough is transmitted on a

line fiber, its energy can be translated to other wavelength section. The signal light is

amplified by translating the energy of shortwave pump light to the long wave signal

light.

E1

E2

h(v-Δv) h(v+Δv)

v-Δv v V+Δv

Figure 2.4-5 Operating Principle of Raman Amplifier



33

In the non-linear medium, the incident photons interact with phonons generated by

molecule oscillation of the medium. The incident photons are scattered by the medium

molecules to low-frequency Stocks photons, and other energies are translated to the

phonons at the same time. Then the molecules implement the transition between

oscillation states.

Stocks frequency shift, depending on the chattering level of photons, determines the

frequency range of SRS.

Vr=Vp-Vs

Where,

Vp: frequency of pump light

Vs: frequency of signal light

For amorphous silica fiber, implement amplification of signal light through SRS in a

relatively wide frequency rang Vp-Vs (40THz).

1. Features of Raman fiber amplifier

1) Based on dozens of kilometers of line fibers, it implements distributed

amplification, with low NF and effective improvement of system SNR.

2) With the same SNR, it can reduce the optical power at the transmitting end and

minimize the non-linear effect.

3) It can generate gain for all the wavelengths, serving as full-band amplifier

(however, it should be divided into C band amplifier and L band amplifier).

· It has flat gain. The gain wavelength range depends on the pump wavelength.

· Since the noise of Raman fiber amplifier reduces with fiber distance increase,

the fiber should be long enough. There is no requirement for the fiber type.

· The pump conversion efficiency is low, so the high-power pump laser source is

required.

· The amplifying gain is low, so it needs to cooperate with the EDFA to form

combined amplifier, in order to compensate the line attenuation and node

insertion loss.

2 Application

If the DWDM system above 40 G only uses EDFA for amplifying, spontaneous


34

emission will accumulate, restricting the overall system performance. Compared

with EDFA, the SRA has such advantages as low noise, not introducing in

additional loss upon removal of pump light, and no transient effect. Therefore, the

combination of EDFA and SRA can form the important optical amplifying

technology for the ultra long-haul transmission system above 40 G.

Besides the reverse pumping distributed Raman amplification, other kinds of

Raman amplification technologies also emerges, such as forward pump and

bidirectional pump Raman amplification, which can provide higher gain and

lower noise figure, achieving the flatness of gain and NF at the same time. The

discrete Raman amplifier taking the Dispersion Compensation Fiber (DCF) as the

gain medium can compensate the dispersion on the transmission links and

implement the total ultra broadband integrated amplification of optical signals. In

addition, it has the potential to adjust the gain slope. In the overall Raman

transmission system, in which the distributed and discrete Raman amplifiers are

used, the continuous gain bandwidth can reach 100 nm. Such system supports the

ultra broadband transmission including the S band and xL band.

Of course, the Raman amplification has its inherent shortcomings. The forward

pump and bidirectional pump Raman amplification has the problem of pump light

Relative Intensity Noise (RIN) transition, which has evident influence on the

noise characteristic of the Raman amplifier. Especially in the transmission fibers

with small dispersion coefficient, such as G.655 fiber, this RIN transition problem

becomes more serious, and it degrades the noise figure of Raman amplifier greatly.

To sum up, the discrete Raman amplifier is not better than the EDFA on the

aspect of economic efficiency and noise figure.

2.4.2.2 Classifications of Raman Amplifier

1) Lumped Raman Amplifier (LRA)

Based on the Stimulated Raman Scattering (SRS) effect, LRA adopts

counter-pumped mode. The detailed implementations are described as follows:

Inject laser light with high power into transmission fiber via the output end of

fiber span. The transmission direction of pump lights is contrary to that of

signal lights, and the wavelength of pump lights is 100nm shorter than that of

signal lights. The materials in high-power pump fiber trigger virtual excited

state, from where electrons transit to ground state to implement the gain of



35

optical signals. Actually, the transmission fiber of LRA itself is the gain

medium. The signals are amplified at the same time being transmitted, leading

to negative noise figure. LRA with low noise figure can effectively overcome

the influence of non-linear effect, such as four wave mixing, and improve the

OSNR of the system.

2) Distributed Raman Amplifier (DRA)

DRA adopts dispersion compensation fiber or high non-linear fiber as the

amplification medium, such as DCF fiber or telluro fiber. At present, the Raman

gain coefficient of DCF fiber increases about 10 times of that of SMF fiber. The

Raman gain coefficient of telluro fiber is 16 times of that of silica fibre, and the

peak is up to 55W/km.

2.5 Supervision Technology

Detection, control and management are basic requirements of all the network

operations. To ensure the secure operation of DWDM systems, the supervision system

is designed as an independent system separated from working channels and devices

physically.

For example, ZTE’s DWDM system uses an independent wavelength (1510 nm) and

depends on no service channel, to ensure that no active amplification is required for the

long distance transmission and improve the reliability. In this way, the supervision

system can monitor all the NE equipment in the system.

2.5.1 Functions of Optical Supervisory Channel (OSC)

Different from the conventional SDH system, the DWDM system with optical

amplifier can supervise and manage EDFAs in the system additionally. Since the EDFA

only amplifies optical signals without electrical signal input. Especially when it is used

as an optical amplifier regenerator, it has no electrical interface connection because no

service signal will be added or dropped on it. This makes it difficult for supervision. In

addition, there is no special byte in the SDH overhead for monitoring the EDFA, so an

electrical signal must be added to monitor the status of EDFA.

The OSC is used to transmit the NE management and supervision information related

to the DWDM system through a wavelength. The information involves the fault alarm,

fault location, quality parameter supervision during operation, the control over backup


36

line upon line interruption and the EDFA supervision etc. In this way, the network

operator can effectively manage the DWDM system.

2.5.2 Requirements for OSC

The DWDM system has the following requirements for the OSC:

1. The OSC should not restrict the optical wavelengths (980 nm and 1480 nm) of

the pump light source in the optical amplifier.

2. The OSC should not restrict the transmission distance between two OLAs.

3. The OSC should not restrict the services on the 1310nm wavelength in the

future.

4. The OSC can still be available upon failure of the OLA.

5. The OSC transmission is bidirectional, which ensures the supervision

information can still be received by the line terminal when one fiber is broken.

6. The segmenting of OSC transmission enables dropping supervision information

or adding new supervision information on each optical amplifier regeneration

station and DWDM system office station.

2.5.3 Implementation of OSC

The implementation principle of OSC is shown in Figure 2.5-1.

OTUT

OTUT OM OBA OPA OD

OTUR

OTUR

Line fiber Internal fiber

OM2 OD2

OSC information OSC information

2

1

2

1

osc osc

OM: Optical Multiplexer

OD: Optical Demultiplexer

OBA/OPA: Optical amplifier

OTUT/OTUR: Optical transponder

Figure 2.5-1 Implementation Principle of OSC

1. Dropping and adding of OSC information



37

As shown in Figure 2.5-1, to ensure that the supervision information transmitted

on the OSC can be dropped or added on each optical amplifier regeneration

station and DWDM system office station without influences from optical

amplifier, it is required to use a 2-wavelength OM (OM2) behind the OBA at the

transmitting end to add the OSC information into the main channel; and use a

2-wavelength OD (OD2) ahead of the OPA at the receiving end to drop the OSC

information.

2. Operating wavelength of OSC

For the DWDM system with line amplifiers, an additional OSC is required,

which should be able to perform adding/dropping with BER as low as possible

in each optical regenerator/amplifier.

According to ITU-T recommendations, a specific wavelength can be used as the

OSC. Such wavelength can be 1,310 nm, 1,480 nm or 1,510 nm when it is out of

the service transmission band, among which the 1,510 nm is preferable.

Since this channel is out of the gain bandwidth of the EDFA (also called as

outband OSC), the supervision signals must be dropped (from optical channel)

ahead of EDFA and be added (to optical channel) behind the EDFA. As shown in

Figure 2.5-1, the OSC is added behind the OBA and dropped ahead of the OPA.

3. Transmission rate of OSC

In actual DWDM systems, most of the information really needing supervision is

the working status of EDFA. So the amount of supervision information is not

huge. In addition, to ensure normal operation of the OSC upon optical amplifier

failure, the receiving sensitivity should be high in order to enable the

supervision channel signals without being amplified covering the maximum

transmission distance of major service signals. Therefore, the working rate of the

OSC is set to 2 Mbit/s.

With the continuous technology development, the OSC rate improves as well.

For example, ZTE’s DWDM equipment can provide supervision rate of 10

Mbit/s or 100 Mbit/s.

4. Frame structure of OSC information

For the supervision system at working rate of 2 Mbit/s, thirty-two 64 kbit/s bytes

are used to carry supervision information, which is transmitted and exchanged in


38

PCM32 frame format.

For the system at supervision rate of 10 Mbit/s or 100 Mbit/s, taking ZTE’s

DWDM equipment as example, the supervision channel adopts 10/100 M

Ethernet technology to encapsulate supervision data in IP packets. Then the

supervision information is transmitted and exchanged in Ethernet data frames.

5. Line coding

The 2Mbit/s supervisory channel adopts Code Mark Inversion (CMI) as the line

code type.

The 10/100 Mbit/s supervisory channel adopts 4B/5B code.

6. OSC protection

If the OSC bidirectional transmission is interrupted because of the total

break-off of the fiber, the NE management system cannot obtain the supervision

information normally. At this time, the backup route, such as the Data

Communication Network (DCN), should be used to transmit supervision

information so as to protect the OSC.

39

3 Relative Technical Standards

Key points

Open DWDM system and integrated DWDM system

Range of DWDM operating wavelength

DWDM specifications

3.1 Open DWDM System and Integrated DWDM System

DWDM system provides open DWDM system and integrated DWDM system,

described as follows:

· Open DWDM system: The transmitting side of the system provides the Optical

Transponder Unit (OTU) to converts the customer signals with non-standard

wavelength into the standard wavelength compliant with ITU-T G.692. The

"Open" means that the DWDM system has no special requirements for the

operating wavelength of input signals. For example, the signals are accessed

through “Open Optical Interfaces” as shown in Figure 3.1-1.

Figure 3.1-1 Open DWDM System


40

· Integrated DWDM system: All the customer signals accessed to the DWDM

system must comply with ITU-T G.692. For example, some signals are accessed

to the DWDM system not through “Open Optical Interfaces” as shown in Figure

3.1-2.

Figure 3.1-2 Integrated DWDM System

3.2 Operating Wavelength Range

The quartz fiber has three common low-loss windows: 850 nm, 1310 nm and 1550 nm,

as shown in Figure 3.2-1.



41

0

0.5

1.0

1.5

2.0

2.5

3.0

800 1000 1200 1400 1600

Wavelength (nm)

Loss (dB/km)

~140THz

~50THz

OH- absorption peak

OH- absorption

peak

OH- absorption peak

O ES C L

O: Original Band E: Extend Band S: Short Band C: Conventional Band L: Long Band

Figure 3.2-1 Low-Loss Windows in Fiber Communication

1. 850 nm window

The wavelength range is 600 nm - 900 nm. It is always used in multi-mode fiber,

and the transmission loss is large (2 dB/km averagely). The 860 nm window is

applicable to short-distance access networks, such as for Fiber Channel (FC)

services.

2. 1310 nm window

The lower limit of available wavelengths in this window depends on the fiber

cut-off wavelength and attenuation coefficient, while the upper limit depends on

the OH absorption peak at 1385 nm. The operating wavelength range is 1260 nm

- 1360 nm. The average loss is 0.3 dB/km - 0.4 dB/km.

The 1310 nm window is applicable to intra-office, short-distance and

long-distance communication of STM-N signals (N=1, 4 or 16).

Multi-longitudinal mode lasers (MLM) and Light Emitting Diodes (LED) can be

adopted as light sources.

Since the broadband optical amplifier working in 1310 nm window is not

available at present, this window is not suitable for the DWDM system.

3. 1550 nm window

The lower limit of available wavelengths in this window depends on the OH


42

absorption peak at 1385 nm, while the upper limit depends on infrared

absorption loss and bending loss. The operating wavelength range is 1460 nm -

1625 nm. The average loss is 0.19 dB/km - 0.25 dB/km.

The loss in the 1550 nm window is the lowest, so it can be applied to

short-distance and long-distance communication of SDH signals. In addition, the

commonly used Erbium-Doped Fiber Amplifier (EDFA) has sound gain flatness

in this window, so the 1550 nm window is applicable to the DWDM system as

well.

The operating wavelength in the 1550 nm window is divided into three parts (S

band, C band and L band), with the wavelength range shown in Figure 3.2-2.

1460 nm 1530 nm 1565 nm 1625 nm

Short bandConventional

bandLong band

1460 - 1530

nm

1530 - 1565

nm

1565 - 1625

nm

Figure 3.2-2 Division of Operating Wavelength in 1550 Window

1) S band (1460 nm - 1530 nm): Since the operating wavelength range of EDFA is

in C band or L band, S band is not used in the DWDM system at present.

2) C band (1530 nm - 1565 nm): It is often used as the operating wavelength area

of DWDM systems under 40 wavelengths (with channel spacing 100 GHz),

DWDM systems under 80 wavelengths (with channel spacing 50 GHz) and

SDH systems.

3) L band (1565 nm - 1625 nm): Operating wavelength area of DWDM systems

above 80 wavelengths. In this case, the channel spacing is 50 GHz.

3.3 Operating Wavelength of DWDM System

3.3.1 Operating Wavelength Area of DWDM Systems

Based on the quantity of multiplexing channel and frequency spacing, the system of 40

wavelengths or below, 80-wavelength system and 160-wavelength system are

introduced respectively as follows.

1. 8/16/32/40/48-wavelength system



43

Operating wavelength range: C band (1530 nm - 1565 nm)

Frequency range: 191.3THz - 196.0 THz

Channel spacing: 100 GHz

Central frequency offset: ±20 GHz (at rate lower than 2.5 Gbit/s); ±12.5 GHz (at

rate 10 Gbit/s)

2. 80/96-wavelength system

Operating wavelength range: C band (1530 nm - 1565 nm)

Frequency range: C band (191.30THz - 196.05THz)


Central channel offset: ±5 GHz

3. 160/176-wavelength system

Operating wavelength range: C band (1530 nm - 1565 nm) + L band (1565 nm -

1625 nm)

Frequency range: C band (191.3THz - 196.05 THz) + L band (186.95 THz -

190.90)


Central frequency offset: ±5 GHz

3.3.2 Operating Wavelength Allocation in DWDM Systems

The operating wavelength of the ZTE DWDM equipment, complying with ITU-T

Recommendation G.692, adopts the specific central wavelength and central frequency

values in the multi-channel system.

1. For a 40-channel DWDM system (C band), Table 3.3-1 lists the

wavelength/frequency allocation in the system. The channel spacing is 100 GHz.

Table 3.3-1 Wavelength Allocation of 40CH on C Band

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

1 C100_1 192.1 1560.61 21 C100_1 194.1 1544.53

2 C100_1 192.2 1559.79 22 C100_1 194.2 1543.73


44

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

3 C100_1 192.3 1558.98 23 C100_1 194.3 1542.94

4 C100_1 192.4 1558.17 24 C100_1 194.4 1542.14

5 C100_1 192.5 1557.36 25 C100_1 194.5 1541.35

6 C100_1 192.6 1556.55 26 C100_1 194.6 1540.56

7 C100_1 192.7 1555.75 27 C100_1 194.7 1539.77

8 C100_1 192.8 1554.94 28 C100_1 194.8 1538.98

9 C100_1 192.9 1554.13 29 C100_1 194.9 1538.19

10 C100_1 193.0 1553.33 30 C100_1 195.0 1537.40

11 C100_1 193.1 1552.52 31 C100_1 195.1 1536.61

12 C100_1 193.2 1551.72 32 C100_1 195.2 1535.82

13 C100_1 193.3 1550.92 33 C100_1 195.3 1535.04

14 C100_1 193.4 1550.12 34 C100_1 195.4 1534.25

15 C100_1 193.5 1549.32 35 C100_1 195.5 1533.47

16 C100_1 193.6 1548.51 36 C100_1 195.6 1532.68

17 C100_1 193.7 1547.72 37 C100_1 195.7 1531.90

18 C100_1 193.8 1546.92 38 C100_1 195.8 1531.12

19 C100_1 193.9 1546.12 39 C100_1 195.9 1530.33

20 C100_1 194.0 1545.32 40 C100_1 196.0 1529.55

Note: C100_1 refers to the first sub-band in C band with the spacing at 100GHz.

2. For a 48/96-channel DWDM system (C band), Table 3.3-2 lists the

wavelength/frequency allocation in the system. The channel spacing is 50 GHz

or 100 GHz.

Table 3.3-2 Wavelength Allocation of 48/96CH on C Band

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

1 C100_2 196.05 1529.16 49 C100_2 193.65 1548.11

2 C100_1 196.00 1529.55 50 C100_1 193.60 1548.51

3 C100_2 195.95 1529.94 51 C100_2 193.55 1548.91

4 C100_1 195.90 1530.33 52 C100_1 193.50 1549.32

5 C100_2 195.85 1530.72 53 C100_2 193.45 1549.72

6 C100_1 195.80 1531.12 54 C100_1 193.40 1550.12

7 C100_2 195.75 1531.51 55 C100_2 193.35 1550.52

8 C100_1 195.70 1531.90 56 C100_1 193.30 1550.92



45

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

9 C100_2 195.65 1532.29 57 C100_2 193.25 1551.32

10 C100_1 195.60 1532.68 58 C100_1 193.20 1551.72

11 C100_2 195.55 1533.07 59 C100_2 193.15 1552.12

12 C100_1 195.50 1533.47 60 C100_1 193.10 1552.52

13 C100_2 195.45 1533.86 61 C100_2 193.05 1552.93

14 C100_1 195.40 1534.25 62 C100_1 193.00 1553.33

15 C100_2 195.35 1534.64 63 C100_2 192.95 1553.73

16 C100_1 195.30 1535.04 64 C100_1 192.90 1554.13

17 C100_2 195.25 1535.43 65 C100_2 192.85 1554.54

18 C100_1 195.20 1535.82 66 C100_1 192.80 1554.94

19 C100_2 195.15 1536.22 67 C100_2 192.75 1555.34

20 C100_1 195.10 1536.61 68 C100_1 192.70 1555.75

21 C100_2 195.05 1537.00 69 C100_2 192.65 1556.15

22 C100_1 195.00 1537.40 70 C100_1 192.60 1556.55

23 C100_2 194.95 1537.79 71 C100_2 192.55 1556.96

24 C100_1 194.90 1538.19 72 C100_1 192.50 1557.36

25 C100_2 194.85 1538.58 73 C100_2 192.45 1557.77

26 C100_1 194.80 1538.98 74 C100_1 192.40 1558.17

27 C100_2 194.75 1539.37 75 C100_2 192.35 1558.58

28 C100_1 194.70 1539.77 76 C100_1 192.30 1558.98

29 C100_2 194.65 1540.16 77 C100_2 192.25 1559.39

30 C100_1 194.60 1540.56 78 C100_1 192.20 1559.79

31 C100_2 194.55 1540.95 79 C100_2 192.15 1560.20

32 C100_1 194.50 1541.35 80 C100_1 192.10 1560.61

33 C100_2 194.45 1541.75 81 C100_2 192.05 1561.02

34 C100_1 194.40 1542.14 82 C100_1 192.00 1561.42

35 C100_2 194.35 1542.54 83 C100_2 191.95 1561.83

36 C100_1 194.30 1542.94 84 C100_1 191.90 1562.24

37 C100_2 194.25 1543.33 85 C100_2 191.85 1562.64

38 C100_1 194.20 1543.73 86 C100_1 191.80 1563.05

39 C100_2 194.15 1544.13 87 C100_2 191.75 1563.46

40 C100_1 194.10 1544.53 88 C100_1 191.70 1563.87

41 C100_2 194.05 1544.92 89 C100_2 191.65 1564.27

42 C100_1 194.00 1545.32 90 C100_1 191.60 1564.68

43 C100_2 193.95 1545.72 91 C100_2 191.55 1565.09

44 C100_1 193.90 1546.12 92 C100_1 191.50 1565.5

45 C100_2 193.85 1546.52 93 C100_2 191.45 1565.91


46

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

Wavelength

No.

Sub-band

Name

Central

Frequency

(THz)

Central

Wavelength

(nm)）

46 C100_1 193.80 1546.92 94 C100_1 191.40 1566.32

47 C100_2 193.75 1547.32 95 C100_2 191.35 1566.73

48 C100_1 193.70 1547.72 96 C100_1 191.30 1567.14

Note: C100_1 and C100_2 respectively refer to the first and the second sub-band in C band with the spacing at 100 GHz. Each sub-band includes 48

wavelengths.

3. For an 80-channel DWDM system in C band consisting of ZTE DWDM

equipment, Table 3.3-3 lists the wavelength/frequency allocation in the system.

The channel spacing is 50 GHz.

Table 3.3-3 Wavelength Allocation of 80CH on C Band

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

1 C50_1 196.05 1529.16 41 C50_1 194.05 1544.92

2 C50_1 196.00 1529.55 42 C50_1 194.00 1545.32

3 C50_1 195.95 1529.94 43 C50_1 193.95 1545.72

4 C50_1 195.90 1530.33 44 C50_1 193.90 1546.12

5 C50_1 195.85 1530.72 45 C50_1 193.85 1546.52

6 C50_1 195.80 1531.12 46 C50_1 193.80 1546.92

7 C50_1 195.75 1531.51 47 C50_1 193.75 1547.32

8 C50_1 195.70 1531.90 48 C50_1 193.70 1547.72

9 C50_1 195.65 1532.29 49 C50_1 193.65 1548.11

10 C50_1 195.60 1532.68 50 C50_1 193.60 1548.51

11 C50_1 195.55 1533.07 51 C50_1 193.55 1548.91

12 C50_1 195.50 1533.47 52 C50_1 193.50 1549.32

13 C50_1 195.45 1533.86 53 C50_1 193.45 1549.72

14 C50_1 195.40 1534.25 54 C50_1 193.40 1550.12

15 C50_1 195.35 1534.64 55 C50_1 193.35 1550.52

16 C50_1 195.30 1535.04 56 C50_1 193.30 1550.92

17 C50_1 195.25 1535.43 57 C50_1 193.25 1551.32

18 C50_1 195.20 1535.82 58 C50_1 193.20 1551.72

19 C50_1 195.15 1536.22 59 C50_1 193.15 1552.12

20 C50_1 195.10 1536.61 60 C50_1 193.10 1552.52

21 C50_1 195.05 1537.00 61 C50_1 193.05 1552.93



47

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

22 C50_1 195.00 1537.40 62 C50_1 193.00 1553.33

23 C50_1 194.95 1537.79 63 C50_1 192.95 1553.73

24 C50_1 194.90 1538.19 64 C50_1 192.90 1554.13

25 C50_1 194.85 1538.58 65 C50_1 192.85 1554.54

26 C50_1 194.80 1538.98 66 C50_1 192.80 1554.94

27 C50_1 194.75 1539.37 67 C50_1 192.75 1555.34

28 C50_1 194.70 1539.77 68 C50_1 192.70 1555.75

29 C50_1 194.65 1540.16 69 C50_1 192.65 1556.15

30 C50_1 194.60 1540.56 70 C50_1 192.60 1556.55

31 C50_1 194.55 1540.95 71 C50_1 192.55 1556.96

32 C50_1 194.50 1541.35 72 C50_1 192.50 1557.36

33 C50_1 194.45 1541.75 73 C50_1 192.45 1557.77

34 C50_1 194.40 1542.14 74 C50_1 192.40 1558.17

35 C50_1 194.35 1542.54 75 C50_1 192.35 1558.58

36 C50_1 194.30 1542.94 76 C50_1 192.30 1558.98

37 C50_1 194.25 1543.33 77 C50_1 192.25 1559.39

38 C50_1 194.20 1543.73 78 C50_1 192.20 1559.79

39 C50_1 194.15 1544.13 79 C50_1 192.15 1560.20

40 C50_1 194.10 1544.53 80 C50_1 192.10 1560.61

Note: C5_1 refers to the first sub-band in C band with the spacing at 50 GHz.

4. For an 80-channel DWDM system in L band consisting of ZTE DWDM

equipment, Table 3.3-3 lists the wavelength/frequency allocation in the system.


Table 3.3-4 Wavelength Allocation of 80CH on L Band

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

1 L50_1 190.90 1570.42 41 L50_1 188.90 1587.04

2 L50_1 190.85 1570.83 42 L50_1 188.85 1587.46

3 L50_1 190.80 1571.24 43 L50_1 188.80 1587.88

4 L50_1 190.75 1571.65 44 L50_1 188.75 1588.30

5 L50_1 190.70 1572.06 45 L50_1 188.70 1588.73


48

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

Nominal

Central

Wavelength

(nm)

Wavelength

No.

Nominal

Central

Frequency

(THz)

6 L50_1 190.65 1572.48 46 L50_1 188.65 1589.15

7 L50_1 190.60 1572.89 47 L50_1 188.60 1589.57

8 L50_1 190.55 1573.30 48 L50_1 188.55 1589.99

9 L50_1 190.50 1573.71 49 L50_1 188.50 1590.41

10 L50_1 190.45 1574.13 50 L50_1 188.45 1590.83

11 L50_1 190.40 1574.54 51 L50_1 188.40 1591.26

12 L50_1 190.35 1574.95 52 L50_1 188.35 1591.68

13 L50_1 190.30 1575.37 53 L50_1 188.30 1592.10

14 L50_1 190.25 1575.78 54 L50_1 188.25 1592.52

15 L50_1 190.20 1576.20 55 L50_1 188.20 1592.95

16 L50_1 190.15 1576.61 56 L50_1 188.15 1593.37

17 L50_1 190.10 1577.03 57 L50_1 188.10 1593.79

18 L50_1 190.05 1577.44 58 L50_1 188.05 1594.22

19 L50_1 190.00 1577.86 59 L50_1 188.00 1594.64

20 L50_1 189.95 1578.27 60 L50_1 187.95 1595.06

21 L50_1 189.90 1578.69 61 L50_1 187.90 1595.49

22 L50_1 189.85 1579.10 62 L50_1 187.85 1595.91

23 L50_1 189.80 1579.52 63 L50_1 187.80 1596.34

24 L50_1 189.75 1579.93 64 L50_1 187.75 1596.76

25 L50_1 189.70 1580.35 65 L50_1 187.70 1597.19

26 L50_1 189.65 1580.77 66 L50_1 187.65 1597.62

27 L50_1 189.60 1581.18 67 L50_1 187.60 1598.04

28 L50_1 189.55 1581.60 68 L50_1 187.55 1598.47

29 L50_1 189.50 1582.02 69 L50_1 187.50 1598.89

30 L50_1 189.45 1582.44 70 L50_1 187.45 1599.32

31 L50_1 189.40 1582.85 71 L50_1 187.40 1599.75

32 L50_1 189.35 1583.27 72 L50_1 187.35 1600.17

33 L50_1 189.30 1583.69 73 L50_1 187.30 1600.60

34 L50_1 189.25 1584.11 74 L50_1 187.25 1601.03

35 L50_1 189.20 1584.53 75 L50_1 187.20 1601.46

36 L50_1 189.15 1584.95 76 L50_1 187.15 1601.88

37 L50_1 189.10 1585.36 77 L50_1 187.10 1602.31

38 L50_1 189.05 1585.78 78 L50_1 187.05 1602.74

39 L50_1 189.00 1586.20 79 L50_1 187.00 1602.17

40 L50_1 188.95 1586.62 80 L50_1 186.95 1603.57

Note: L50_1 refers to the first sub-band in L band with the spacing at 50 GHz.



49

5． For a 160-channel DWDM system in C+L band consisting of ZTE DWDM

equipment (80 wavelengths in each band), Table 3.3-3 and Table 3.3-4 list the

wavelength/frequency allocation in the system. The channel spacing is 50 GHz.

6． For a 176-channel DWDM system in C+L band consisting of ZTE DWDM

equipment (96 wavelengths in C band, while 80 wavelengths in L band), Table

3.3-2 and Table 3.3-4 list the wavelength/frequency allocation in the system.


3.4 Main Performance Indices

1) Channel spacing

Channel spacing means the nominal frequency difference between two adjacent

multiplexing channels, covering uniform channel spacing and non-uniform

channel spacing. At present, uniform channel spacing is used mostly.

The minimum channel spacing of the DWDM system is the integer times of 50

GHz.

· When the multiplexing channels are 8 wavelengths, the channel spacing is 200

GHz.

· When the multiplexing channels are 16/32/40/48 wavelengths, the channel

spacing is 100 GHz.

· When the multiplexing channels are above 80 wavelengths, the channel spacing

is 50 GHz.

Smaller channel spacing requires higher resolution of the OD and means more

multiplexing channels.

2) Nominal central frequency

It refers to the central wavelength (frequency) corresponding to each

multiplexing channel in the DWDM system.

For example, when the multiplexing channels are 16/32/40 wavelengths, the

central frequency of the first wavelength is 192.1 THz and the channel spacing

is 100 GHz. The frequency increases in ascending order.

3) Central frequency offset


50

It is also called frequency offset. It refers to the offset between the actual

working central frequency of the multiplexing optical channel and nominal

central frequency.

According to the national standards, in the system with frequency spacing of

100 GHz, the maximum central frequency offset is ±20 GHz (about ±0.16 nm)

when the rate is below 2.5 Gbit/s, and it is ±12.5 GHz when the rate is 10 Gbit/s.

For the system with frequency spacing as 50 GHz, the maximum central

frequency offset is ±5 GHz.

The maximum central frequency offset is the value which can still be met when

the designed life cycle of the system expires, with temperature, humidity and

other factors taken into consideration.

4) Dispersion tolerance

Dispersion reflects the spreading of the optical pulse during the transmission in

the fiber.

The pulse spreading will result in decreased extinction ratio of signal pulse at

the receiving end, that is, the electrical level of bit “1” and bit “0” are close to

each other, which may lead to mistaken judgment of the receiver. To avoid bit

errors, it is required to take proper measures to compensate the optical pulse

spreading in the fiber transmission process, for the pulse spreading will be more

and more serious with the increasing of transmission distance.

The requirement of DWDM system for the fiber chromatic dispersion

coefficient is basically that of a single multiplexing channel signal for fiber

chromatic dispersion coefficient. In addition, since the passive regenerating

distance of the DWDM system is much greater than that of a single SDH system,

the dispersion tolerance distance of the system light source must be prolonged.

5) Receiver sensitivity

The receiver sensitivity refers to the minimum average receiving optical power

on the OTU input port when the input signals are located in the 1550 nm

window and the bit error rate reaches 10-12

.

6) Overloaded optical power

The overloaded optical power refers to the maximum average receiving optical

power on the OTU input port when the input signals are located in the 1550 nm



51

window and the bit error rate reaches 10-12

.

7) Optical power unit

W: optical power unit. Since the optical energy during the optical transmission

process is small, we usually use mW to measure.

dBm: it is an absolute value of 1mW

P dBm=10*log10(P/P0) =10*log10(pmW/1mW)

dB: a relative value. It is the power value of P1 relative to that of P2

P1-P2 dB=10*log10(P1/P2) =10*log10(P1/P0)- 10*log10(P2/P0)

8) Loss

Loss refers to the energy loss caused by passive devices. As a relative value, it

usually selects the output power of passive device as the reference point, and the

ratio of input power to output power is the loss of this device. The fiber loss is

proportional to the length of the fiber, that is, fiber loss is cumulated.

9) Gain

Gain refers to the energy increase caused by amplifiers. As a relative value, it

usually selects the input power of amplifier as the reference point, and the ratio

of output power to input power is the gain of this device.

10) OSNR

OSNR refers to the ratio of optical signal power to noise power. It is an

important parameter for estimating and measuring the system bit error

performance, engineering design and maintenance.

Take the OSNR formula at the receiving end of a DWDM system as an example.

The calculation formula is:

OSNR = Pout - 10 ㏒ M - L + 58 - NF - 10 ㏒ N

Where,

Pout: the input optical power (dBm)

M: Number of multiplexing channels of the DWDM system

L: Loss between any two optical amplifiers, that is, section loss (dB)

NF: Noise figure of EDFA (dB)


52

N: Number of optical amplifiers between optical multiplexer and optical

demultiplexer of the DWDM system.

The formula shows that when the other parameters keep unchanged, greater line

loss leads to lower OSNR, which means decreased transmission quality of the

optical line.

11) Modulation and non-modulation

The process to upload digital stream to the laser for transmitting is called

modulation. The output optic from the modulation process is modulation optic.

When the laser does not receive regular clock signals from receiver, the output

optic is non-modulation optic with irregular bit stream, whose output power is

unstable.

12) Optical interface code

The application code of WDM system is nWX-Y.Z defined in ITU-T G.692.

N: Number of multiplexing channel in the system

W: segment type, such as L, V and U

X: Number of area between multiplexer and demultiplexer in WDM system

Y: STM level of multiplexing channel signals

Z: fiber type, Z=2, 3, 5

Z=2, G.652 fiber

Z=3, G.653 fiber

Z=5, G.655 fiber

53

Appendix A Abbreviations

Abbreviation Full Name

AFR Absolute Frequency Reference

AFEC Advanced FEC

AIS Alarm Indication Signal

APR Automatic Power Reduction

APS Automatic Protection Switching

APSD Automatic Power Shutdown

APSF Automatic Protection Switching for Fast Ethernet

ASE Amplified Spontaneous Emission

AWG Array Waveguide Grating

BER Bit Error Ratio

BLSR Bidirectional Line Switching Ring

BSHR Bidirectional Self-Healing Ring

CDR Clock and Data Recovery

CMI Code Mark Inversion

CODEC Code and Decode

CPU Center Process Unit

CRC Cyclic Redundancy Check

DBMS Database Management System

DCC Data Communications Channel

DCF Dispersion Compensation Fiber

DCG Dispersion Compensation Grating

DCN Data Communications Network

DCM Dispersion Compensation Module

DDI Double Defect Indication

DFB-LD Distributed Feedback Laser Diode

DSF Dispersion Shifted Fiber

DGD Differential Group Delay

DTMF Dual Tone Multi-Frequence

DWDM Dense Wavelength Division Multiplexing

DXC Digital Cross-connect

EAM Electrical Absorption Modulation

ECC Embedded Control Channel

EDFA Erbium Doped Fiber Amplifier

EFEC Enhanced FEC


54


EX Extinction Ratio

FDI Forward Defection Indication

FEC Forward Error Correction

FPDC Fiber Passive Dispersion Compensator

FWM Four Wave Mixing

GbE Gigabits Ethernet

GUI Graphical User Interfaces

IP Internet Protocol

LD Laser Diode

MDI Multiple Document Interface

MCU Management and Control Unit

MOADM Metro Optical Add/Drop Multiplexer Equipment

MBOTU Sub-rack backplane for OTU

MQW Multiple Quantum Well

MSP Multiplex Section Protection

MST Multiplex Section Termination

NCP Net Control Processor

NDSF None Dispersion Shift Fiber

NE Network Element

NNI Network Node Interface

NMCC Network Manage Control Center

NRZ Non Return to Zero

NT Network Termination

NZDSF Non-Zero Dispersion Shifted Fiber

OA Optical Amplifier

OADM Optical Add/Drop Multiplexer

OBA Optical Booster Amplifier

Och Optical Channel

ODF Optical fiber Distribution Frame

ODU Optical Demultiplexer Unit

OGMD Optical Group Mux/DeMux Board

OHP Overhead processing board

OHPF Overhead Processing Board for Fast Ethernet

OLA Optical Line Amplifier

OLT Optical Line Termination

OMU Optical Multiplexer Unit

ONU Optical Network Unit

OP Optical Protection Unit

OPA Optical Preamplifier Amplifier

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55


OPM Optical Performance Monitor

OPMSN Optical Protect for Mux Section (without preventing resonance switch)

OPMSS Optical Protect for Mux Section (with preventing resonance switch）

OSC Optical Supervisory Channel

OSCF Optical Supervision channel for Fast Ethernet

OSNR Optical Signal-Noise Ratio

OTM Optical Terminal

OTN Optical Transport Network

OTU Optical Transponder Unit

OXC Optical Cross-connect

PDC Passive Dispersion Compensator

PMD Polarization Mode Dispersion

PDL Polarization Dependent Loss

RZ Return to Zero

SBS Stimulated Brillouin Scattering

SDH Synchronous Digital Hierarchy

SDM Supervision add/drop multiplexing board

SEF Severely Error Frame

SES Severely Error Block Second

SFP Small Form Factor Pluggable

SLIC Subscriber Line Interface Circuit

SMCC Sub-network Management Control Center

SMT Surface Mount

SNMP Simple Network Management Protocol

SPM Self-Phase Modulation

SRS Stimulated Raman Scattering

STM Synchronous Transfer Mode

SWE Electrical Switching Board

TCP Transmission Control Protocol

TFF Thin Film Filter

TMN Telecommunications Management Network

VOA Variable Optical Attenuator

WDM Wavelength Division Multiplexing

XPM Cross-Phase Modulation

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Appendix B New Optical Fiber Types

1. Full-wave fiber

The full-wave fiber, water peak free fiber, eliminates the appended water peak

attenuation caused by the OH- ions by eliminating OH

- ions near the 1385 nm

wavelength. In this way, the fiber attenuation is only determined by the internal

scattering loss of the silicon glass.

Full-wave fiber is numbered as G.652 C&D in ITU-T Recommendations. It is

one kind of G.652 fiber. Its full name is wavelength-expanded dispersion

non-shifted single-mode fiber.

The attenuation of the full-wave fiber becomes flat at the band of 1310 nm-

1600 nm. As internal OH-

ions are already eliminated, no water peak

attenuation will occur even when the fiber is exposed to hydrogen gas. It has

the long-term attenuation reliability.

The full-wave optical fiber can provide a complete transmission band from

1280 nm to 1625 nm. The available wavelength range is about 1.5 times of the

wavelength range of ordinary fibers.

2. Real-wave fiber

The real-wave fiber is a kind of non-zero dispersion shifted single-mode fiber

(G.655 fiber) widely used at present. Its fiber characteristics are similar to those

of G.655 fiber. The zero dispersion point is in short-wavelength area below

1530 nm. In 1549 nm - 1561 nm band, the dispersion coefficient is

2.0ps/nm·km - 3.0ps/nm·km.

The real-wave fiber has small dispersion slope and dispersion coefficient with

the capability of tolerating higher non-linear effect. It is applicable to

large-capacity optical transmission systems, and thus reducing the network

construction cost.

3. Fiber with large effective fiber core are

It also belongs to non-zero dispersion shifted single-mode fiber (G.655 fiber).

Essentially, it improves non-linear resistance capability of the system.


58

The main performance of super-speed system is limited by dispersion and

non-linear effect. Usually, dispersion can be eliminated through dispersion

compensation. But the non-linear effect cannot be eliminated through linear

compensation. The effective area of the fiber determines the fiber non-linear

effect. Larger effective area means higher optical power affordable, that is,

better resistance to non-linear effect.

Transmission_I_06_200909 Work Principle of DWDM 62P

Documents

orientation of dwdm

background of dwdm

overview of dwdm technology

basic structure of dwdm

basic concepts of dwdm

optical transponder

cwdm technology

transmission network